The invention relates to a master cylinder according to the preamble of claim 1, as is disclosed, for example, by DE-A-36 19 793. Such master cylinders are installed in millions in motor vehicles and are therefore generally known. They are usually coupled to a brake booster, which increases the actuating force introduced by a driver and thus ensures that the driver is able to apply high braking forces with a relatively low actuating force. The force that acts on the master cylinder during an actuation is accordingly composed of a foot force component, which is applied by the driver, and a component which is applied by the brake booster. The brake booster is often a vacuum brake booster.
Conventionally, brake boosters are dimensioned such that the maximum actuating force that can be applied by an average driver and increased by the brake booster is sufficient to lock all the wheels of a vehicle, even given high frictional values between wheel and roadway. The braking force distribution of a vehicle is normally designed in such a way that considerably more braking force is fed to the front wheels of the vehicle than to the rear wheels, in order to avoid premature locking of the rear wheels, which is favored by the relieving of load on the rear wheels on account of the dynamic axle-load displacement occurring during braking. For some time, many vehicles have also been equipped with antilock braking systems, which prevent the vehicle wheels locking during sharp braking and thus keep the vehicle steerable. Recently, such antilock braking systems have been used not only during braking but also during the usual driving operation, in order to prevent critical driving states, or at least to reduce their criticality by means of the targeted braking of individual vehicle wheels, or to avoid drive wheels spinning on slippery ground.
Expanded systems of this type have become known under the terms vehicle dynamics control system (VDC system) and traction control system (TC system). The last-mentioned systems are able to brake individual vehicle wheels even if the driver of the vehicle does not actuate the brake. Of course, this presupposes that the brake booster cooperating with the master cylinder can be actuated independently of the driver, in order, if necessary, i.e. for example upon detecting a driving state that is becoming critical or a driving wheel that is spinning, to be able to exert an actuating force on the master cylinder. In this connection, the problem occurs that the actuating force that can be applied by the brake booster on its own is under certain circumstances no longer adequate to produce a braking force that is necessary for braking individual vehicle wheels up to the locking limit. Although this problem can be solved to a certain extent by using correspondingly larger-dimensioned brake boosters, this rapidly reaches the limits, in particular in the case of relatively compact motor vehicles, on account of the installation space, which is limited there. To make things more difficult, the use of a VDC system takes place not during braking but during normal driving operation, where for example the dynamic axle-load displacement which relieves the load on the rear wheels does not occur. Within the context of control by the VDC system, therefore, in order to brake a rear wheel up to the locking limit, a considerably higher braking force is required than even in the case of sharp operational braking. The dynamic wheel load of the front wheel on the outside of a curve can also increase, as a result of the centrifugal force on the vehicle bodywork, which tends to the outer rim of the curve, such that the braking force necessary in order to lock said wheel lies above that which would be required to lock it during straight-line braking. This is similarly true for braking spinning, driven rear wheels within the context of control via a TC system. In particular in the case of vehicles that are equipped with VDC systems, but also in the case of those having a TC system, it is therefore possible, in normal driving operation, for states to occur in which the actuating force that is provided solely by the brake booster and acts on the master cylinder is no longer adequate to produce the braking pressure which is demanded by the VDC system or by the TC system in order to avoid or reduce the criticality of a critical driving state.